1. Field of the Invention
The present invention relates to a biosensor, and more particularly to a biosensor with multi-channel A/D conversion and a method thereof.
2. Description of the Prior Art
In recent years, various kinds of biosensors utilizing a specific catalytic action of enzymes to be used for clinical purposes have been developed. Most valuable use of such biosensors may be made in the area of e.g. diabetes treatment where it is vital for patients to keep their blood glucose concentration (“blood sugar level” below) within a normal range. For an inpatient, the blood sugar level can be kept normal under the supervision of the doctor. For an outpatient, self-control of the blood sugar level is an important factor for treatment in lack of doctor's direct supervision.
The self-control of the blood sugar level is achieved through a diet, exercise and medication. These treatments may often be simultaneously employed under the supervision of the doctor. It has been found that the self-control works more effectively when the patient himself is able to check whether or not his blood sugar level is within the normal range.
Recently, blood sugar determining instruments have been used for self-checking of blood sugar level. For example, U.S. Pat. No. 6,349,230 provides a blood sugar determining instrument, as shown in FIG. 1, which mainly includes a main detecting unit 10 and a chip 12 for blood sugar measurement. As shown in FIG. 2, the chip 12 includes a strip-like substrate 122 provided in its front portion with an electrode section 1221. The electrode section 1221 is covered by a reaction layer 124, a spacer 126 and a cover sheet 128. The electrode section 1221 is provided with an operational terminal 1222 and a counterpart terminal 1224 surrounding the operational terminal 1222. The operational terminal 1222 and the counterpart terminal 1224 are electrically connected to lead terminals 1226 and 1228, respectively, which are formed on a base end portion of the strip-like substrate 122. The reaction layer 124, which covers the electrode section 1221, contains potassium ferricyanide and an oxidase such as glucose oxidase.
The blood sugar determining instruments may be used in the following manner. A patient pricks his or her own skin with e.g. a lancet for oozing blood. Then, the oozed-out blood is caused to touch the tip of the chip 12 plugged into the main detecting unit 1. The blood is partially sucked into the reaction layer 124 by capillary action. The reaction layer 124 disposed above the electrode section 1221, is dissolved by the blood, which starts an enzyme reaction, as the following formula:

Potassium ferrocyanide is produced in an amount corresponding to the glucose concentration. After a certain period of time, a predetermined voltage Vref is applied on the chip 12 to electrochemically oxidize potassium ferrocyanide to release electrons. A response current is generated and passes through the operational terminal 1222. The response current is proportional to the concentration of potassium ferrocyanide produced by the enzyme reaction or to the concentration of the glucose. Therefore, the blood sugar level can be known by measuring the response current.
FIG. 3 is a schematic diagram of a control circuit of the blood sugar determining instrument of FIG. 1, in which the electrode section 1221 of the chip 12 can be regarded as a resistor Rs. The voltage Vref to be applied can be provided by a battery. The response current I generated by the chip 12 decays as time progresses to form a time-dependent discharge curve corresponding to the glucose concentration of the blood. Moreover, the response current I of each sampling time of the time-dependent discharge curve is converted to an output voltage Vout by a current/voltage converter 30 formed of an operational amplifier 310 having an amplification resistance Rf. As a consequence, the response current I decaying as time progressing forms a voltage-time discharge curve, as shown in FIG. 4. Each voltage of each sampling time of the voltage-time discharge curve is converted to a set of digital signals by a single-channel A/D converter 32. A microprocessor 34 reads the digital signals output from the single-channel A/D converter 32, and calculates the glucose concentration of the blood in accordance with the digital signals. A reading of the glucose concentration is displayed on a display such as a liquid crystal display (LCD) 36.
The voltage-time discharge curve shown in FIG. 4 is sampled in each sampling interval, and then sent to the single-channel A/D converter to convert to a set of digital signals. However, the single-channel A/D converter has a limited resolution due to the design itself. As a result, the resolution of the detection of the conventional blood sugar instrument cannot be improved.
Accordingly, it is an intention to provide an improved biosensor, which can overcome the drawback of the conventional one.